As my next-but-one DIY project (I tend to plan fast and
build slowly!), I'm starting to think about building a set
of five smallish HT speakers and a subwoofer for our first
HT setup. I doubt if I'll start building for a year or
so, but I like to play with ideas in my head in the
meantime. I have an almost complete plan so far, but there
are still one or two holes in it.

Plan: five satellites with shielded drivers, probably with
5-6 - inch woofers. Active drive for each speaker, using a
pair of LM1875 amplifier chips and crossovers designed by
me. Satellites wall-mounted, except for centre channel,
which will be placed on a shelf over the TV in an alcove.
Selectable RCA unbalanced and XLR balanced inputs for each.
One active subwoofer placed in said alcove under TV. I have
a fair amount of experience in designing amplifiers,
crossovers and speakers, so this presents an interesting
challenge but little in the way of panic.

What are the snags, then?

Firstly, in contrast to my present music-only setup, with
semi-active three-way speakers in the normal stereo
arrangement, a 5.1 channel active HT system requires each of
six speakers to be driven both by a line-level signal input,
but also to be supplied with its own mains AC supply. The
rear speakers, in particular, will be mounted on wall
brackets behind the listening position. What is the best way
to get the line-level signal and the mains supply across
from the opposite side of the room? Is there a good way to
avoid long trailing leads? We are likely to redecorate the
room within the next year, so cable conduits within the wall
or cables routed under the floor aren't completely out of
the question, but I'm not sure about mains outlets at head
height. Is there a suitable connector for floating mains
leads?

Secondly, I'm after recommendations for good 16-18cm
shielded midbass and treble drivers. My main speakers use
the excellent Scan-Speak 9300 tweeters, but since the 5.1
system will need five of them, I'll be looking for something
a little cheaper, and similar expenditure on the midbass. I
will also need good documentation of frequency response
to help the crossover design.

Electrical outlets sort of goes beyond the scope of the forum, so I won't attempt to answer that.

As for the line level cabling, You could use a wireless system. I have had very little luck with getting reasonable quality from them. Personally, a digital line for each speaker seems the best. Although $90 3 foot cables exist for digital coax, you can use almost any cable which would usually suffer from major static for digital with good results. The downside is needing a DAC on the speaker end. HT setups usually use digital on the DVD player side of things so that's not as much of an issue. Wireless is also very expensive compared to the alternatives. Hope this helps.

Firstly, on the cabling side of things... The digital option would deliver the best sound quality but will cost a MINT. Something you are trying to avoid I think.

Personally I would stick all you lovely little LM1875s and crossovers in a single box and chuck it under your 5.1 decoder. Cable direct to the speakers from there. This sidesteps 2 issues:
- getting the mains to each speaker.
- hum pickup on long line level interconnects- particularly irritating during those quieter passages (you could send full line level out to each l/s and attenuate appropriately at point of use to improve SNR but this introduces a control problem - 10 remote attenuators).
On the practical side of things I have seen trunking installed INTO skirting board as a cable management system. I've not tried it personally but it looks easyish to do and doesn't look bad at all if properly finished.

On recommendations for drive units I'd go for VIFA. They do a good range of shielded drivers and their P17 170mm polypropelene drivers are well regarded.

First of all, as Simon guessed, digital is out - I'm happy
with the technology I mentioned in my post, but I have no
experience with ADC/DACs and the cost would be pretty high.

The trouble with having the amplifiers on the shelf with the
rest of the system is that I'm set on active crossovers, and
having two runs of speaker cable for each speaker is nearly
as bad as a single run of line level interconnect and a
mains cable (and worse in the sense of increasing cable
resistance, capacitance and inductance). Balanced
connections should certainly help on the hum front, and I'm
intending to put a balanced input on the speakers, though
the AV processors I can afford almost certainly won't have
balanced outputs (so I guess I'd have to instal balanced converters).

As far as drivers go, I'm indeed seriously considering the
Vifa range, but the smaller ones like the P13WH that Lynn
Olsen uses in the Ariel and ME2. Actually it's not
immediately clear from the Vifa webpages what the cone
materials are or which drivers are shielded - I'm not sure
whether the M13SG-09 is otherwise identical. I've also
looked at the SEAS polypropylene drivers, which look very
like the one ProAc use in their Response One SC, though
these are more expensive than the Vifas. I'm not sure about
the tweeter - I'd like to use the ScanSpeak D2010, as also
used in the Response One, but as far as I know it's not
magnetically shielded. Any other suggestions?

If you use a set up like this, you likely can avoid hum issues well enough. You could even use ethernet cable or similar to get a cheap decent cable that is built to reject noise as its reason for existing.

Hiding the AC cables is harder. You mentioned running the mains from the other side of the room. Do you not have outlets near your surround locations?

I'm starting to think that I'm going to have to instal a
balanced driver after the processor outputs - I'm perfectly
happy to put in balanced inputs in the speakers, but the
sort of processor I can afford is unlikely to come with
balanced outputs. The only problem is that a standalone
driver means even MORE mains cables!

Yes, I could instal mains outlets behind the listening seat.
What I'd like to do is to have all six of the speakers
sharing a single on/off switch. This means either feeding
them all from a single "Hydra"-type mains plug, or using
a relay in each speaker for switch-on. Either way means more
cabling...

My 'reference' tweeter (I think it makes the 'right sounds'... smooth, detailed and is non fatiguing) would be the Scanspeak D2905/95000. However it does fail both of our 'tests' : its not shielded; its not particularly cheap.

I have heard that Morel's tweeters are akin to the Scanspeak sound, although I have never actually listened to them myself.

Morel website advertises MDT39, MDT40 & DMS30,32.

Do you have any experience of these drivers
Can anybody else comment on how they stack up versus S/S D2905 or D2010 range ??